The production of maleic anhydride is an important process in the fields of petrochemicals and polymer feedstocks. Maleic anhydride is produced, inter alia, by the selective oxidation of n-butane, available for example from petroleum refinery streams. See e.g., U.S. Pat. No. 4,351,773. The Vanadium-phosphorus-oxide (VPO) system is a well known commercial catalyst for the partial oxidation of C.sub.4 hydrocarbons, particularly n-butane, to maleic anhydride. The VPO system is important as a highly selective and active catalyst. The most effective catalyst, vanadyl pyrophosphate, is produced during the pyrolysis of vanadyl (IV) hydrogen phosphate hemihydrate (VHP). The catalyst precursor, VHP, is converted to the pyrophosphate catalyst by calcination in nitrogen, in the absence of oxygen, at ca. 823.degree. K. The structural similarity between vanadyl pyrophosphate and its pyrolitic precursor is manifested in the presence of vanadyl dimers, V.sub.2 O.sub.8, in both structures connected through corners by P.sub.2 O.sub.7 or HPO.sub.4 groups, respectively.
A variety of such catalysts and methods for their production have been described.
U.S. Pat. No. 3,864,280 describes a crystalline phosphorus-vanadium mixed oxide hydrocarbon oxidation catalyst in which the vanadium has an average valence of from +3.9 to +4.6, the P:V ratio is from 0.9:1 to 1.8:1, and the B-phase content is at least 25%.
U.S. Pat. No. 4,043,943 describes forming a phosphorus-vanadium-oxygen precursor by calcining an intermediate obtained by reacting a vanadium compound with phosphorus compound in an oxygen-containing solvent containing less than 20% water.
U.S. Pat. No. 4,100,106 describes a process in which a water precipitated salt complex formed from tetravalent vanadium salt and orthophosphoric acid is calcined at a temperature of at least 300.degree. C.
U.S. Pat. No. 4,116,868 describes a catalyst prepared by calcining a phosphorus-vanadium-oxygen precursor formed in the presence of a small amount of surfactant.
U.S. Pat. No. 4,132,670 describes a crystalline phosphorus-vanadium mixed oxide hydrocarbon oxidation catalyst in which the vanadium has an average valence of from +4.0 to +4.5, the V:P ratio is about 1:1, and the intrinsic surface area is from 1 m.sup.2 /g to 10 m.sup.2 /g.
U.S. Pat. No. 4,288,372 describes a catalyst similar to that set forth in U.S. Pat. No. 3,864,280 but having a surface area greater than 10 m.sup.2 /g and containing lanthanum as a promoter.
U.S. Pat. No. 4,337,174 describes forming a phosphorus-vanadium-oxygen precursor which, prior to calcining, is heated at 130.degree. to 170.degree. C. and dried.
U.S. Pat. No. 4,418,003 describes the formation of a phosphorus-vanadium oxide catalyst by reacting a vanadium compound with phosphorus pentoxide in an acidic alcoholic medium.
U.S. Pat. No. 4,769,477 describes an attrition-resistant phosphorus-vanadium oxide catalyst incorporating silicon dioxide.
Various other catalysts are specified in, for example, U.S. Pat. Nos. 4,062,873, 4,333,853, and 4,562,268.
According to the present invention, vanadyl phosphonates, (VO)C.sub.n H.sub.2n+1 PO.sub.3.1.5H.sub.2 O represent a new class of precursors for the production of vanadyl pyrophosphate. Based upon the behavior of magnetic susceptibility at low temperatures, the vanadyl phosphonates fall into one of two structural categories: "type A" with vanadyl-phosphorus-oxygen connectivities displaying corner sharing vanadyl octahedra and "type B" where face sharing vanadyl dimers give rise to antiferromagnetic exchange interactions at low temperatures.
There is a close structural relationship between vanadyl (IV) hydrogen phosphate hemihydrate and "type B" phosphonates based upon the presence of the face sharing vanadyl dimers. The structure of the phosphonate is determined by packing constraints on the alkyl group, C.sub.n H.sub.2n+1. When the cross sectional area of the alkyl group exceeds 36 .ANG..sup.2 the more open "type A" structure results. Of the "type B" phosphonates, three alkyl phosphonates, VORPO.sub.3.solvent where R is methyl, ethyl, and propyl, exhibit the same face sharing vanadyl dimer observed in layered vanadyl (IV) hydrogen phosphate hemihydrate. See e.g., J. W. Johnson et al. EP 134 157 A2, 1985.
The present invention is a result of the discovery of (1) a new vanadyl phosphate with n=0, vanadyl phosphite and (2) the structural similarity of both the "type B" phosphonates and vanadyl phosphite and the conventional VHP catalyst precursor and the subsequent search for a method of converting these phosphonates to vanadyl pyrophosphate. The present invention relates to the synthesis of vanadyl phosphite and to the identification of new routes for converting the "type B" phosphonates to VPO catalysts that exhibit higher selectivity and conversion in the n-butane oxidation than found with catalysts prepared from the standard VHP precursor.
Thus, a catalyst precursor which can be directly added to the oxidation reaction and which has a low conversion temperature, high selectivity and high conversion for n-butane would be desirable. The compounds of the present invention exhibit higher selectivity and higher conversions than conventional organic catalysts, and their use results in a more efficient process for the production of maleic anhydride.